1. Field of the Invention
The present invention relates to field effect transistors and particularly to ion-sensitive field effect transistors as well as to a method for producing the same.
2. Description of the Related Art
Ion-sensitive field effect transistors (ISFET) serve as detection elements, for example in measuring a pH value, in measuring ionic concentrations or special substance concentrations in solutions of various compositions and conductivities. Areas of application of ion-sensitive field effect transistors for the continuous detection of concentrations are process measurement technology, analytical chemistry, industrial process monitoring or environment monitoring, wherein measurements are usually performed in aqueous solutions or organic mixtures. Aspects of particular importance are a highly precise concentration detection and a minimum long-term drift of the sensor in connection with acceptable costs.
Measurements of ionic concentrations in aqueous media are traditionally performed with glass electrodes both in process measurement technology and in laboratory measurement technology. In many aggressive media, it is possible to operate the glass electrodes stably, but in strongly alkaline solutions, their stability is limited.
Further, glass electrodes are usually not employed in hydrofluoric acid. The adjusting of the measurement value occurs very slowly. When they are manufactured, glass electrodes require a high percentage of manual labor making them expensive. Further, the area of application of glass electrodes is limited, because they produce chippings when breaking. For example, it is not possible to employ glass electrodes in food technology, because the chippings that result from breaking represent dangerous foreign bodies in food. Glass electrodes are preferably used in process measurement technology.
Further, due to technical reasons, particularly because a correspondingly large internal buffer volume is required, analogously to a reference electrode, with a sufficiently designed internal drain, mostly an Ag/AgCl arrangement, there is no possibility of efficient miniaturization of the glass electrodes. In the case of an ion-sensitive field effect transistor, the internal buffer and this second drain system are eliminated.
In addition, due to the necessary glass membrane thickness, the pH measurement system based on glass electrodes is a system with a high impedance and thus sensitive to environmental interferences. Therefore, the measurement lines should always be shielded and the distance of the electrodes should be kept at a minimum.
In contrast to the glass electrodes, the use of ion-sensitive field effect transistors represents a break-proof alternative to the ion-sensitive measurement in liquids so that they may be employed in areas where there are required additional operational safety requirements, such as in food technology. The application of the ion-sensitive field effect transistors for ionic concentration measurement, particularly for the measurement of the pH value, has been known for a long time, see for example P. Bergveld, IEEE Trans. Biomed. E17 (1979), p. 70. These sensors are further suitable for the miniaturization of the measurement system, the production of integrated systems and for a cost-effective production, and are superior to the conventional glass electrode in these respects.
Typically, when measuring with an ion-sensitive field effect transistor, its gate is brought in contact with the measured fluid. A change in potential at the gate caused by a change of an ionic concentration in the measured fluid results in a measurement signal. As the gate comes in direct contact with the measured liquid during the measurement, gate materials have to be used for the application in aggressive media, when high long-term stability and/or a low drift are required, which are resistant to the respective measured medium for long periods of time.
For producing hydrogen ion-sensitive layers, various materials such as Si3N4, Al2N3, ZrO2, Ta2O5 and diamond-like carbon (DLC) have already been examined. For descriptions of such sensors, see, for example, Van der Schoot et al., Sensors & Actuators 4 (1983), p. 267, D. Sobczynska et al., Sensors & Actuators 6 (1984), p. 93, M. Klein et al., VDI-Berichte 509 (1983), p. 275, and T. Sakai et al., Internat. Electron Devices Meeting, Techn. Digest (1987), p. 711.
Ion-sensitive field effect transistors with a gate of Si3N4 are suitable for use in the above requirements only in a limited way, because the gate of Si3N4 is subjected to a high drift and exhibits low long-term stability. Further, the ion-sensitive field effect transistors with a gate of Si3N4 cannot be used in an aggressive media for longer periods of time.
Compared to Si3N4 as gate material, the use of metal oxides as gate material allows to achieve improved properties of ion-sensitive field effect transistors with respect to pH sensitivity, long-term stability, photosensitivity and drift.
Due to the crystalline structure of the metal oxides after thermal stress, the settling times of these sensors are larger as compared to the ion-sensitive field effect transistors of usually amorphous Si3N4. Ion-sensitive field effect transistors with a metal oxide gate further have the disadvantage that they do not have sufficient resistance to alkaline solutions at higher temperatures and hydrofluoric acid. Further, thermal treatments of metal oxides are performed in prior art for improving the chemical stability of an ion-sensitive field effect transistor on metal oxide basis. This forms crystalline modifications, causing, however, weak spots at the grain boundaries, which, in turn, favor ionic indiffusion which may increase drift or degrade photosensitivity.
Recent developments use ion-sensitive field effect transistors in which amorphous diamond-like carbon (DLC) is used as gate material, which has a high percentage of sp3-hybridized carbon bonds. Such systems have very high chemical resistance, particularly in hydrofluoric acid. One disadvantage of using amorphous diamond-like carbon is, however, that layers of this material have a high internal stress which reduces layer adhesion and may result in the layers peeling off and the consequent destruction of the ion-sensitive field effect transistor. Furthermore, phase boundaries may result in long settling times of the measurement signal. Therefore, these ion-sensitive field effect transistors are not suitable for long-term use in all areas of application.
For the production of stable transistors, the layer materials used in prior art require a double-layer system in the channel consisting of SiO2 and the sensitive layer to achieve a stable interface to silicon provided with minimal charge centers. This is associated with an increased production effort for the sensor chips and, due to the critical interfaces, a reduced yield.